Variations in Authigenic Mineralogy and Sorptive Zeolite Abundance at Yucca Mountain, Nevada, Based on Studies of Drill Cores USW GU-3 and G-3

摘要

Drill Hole USW GU-3 was cored continuously from the surface to a depth of 2637.0 ft (803.8 m) beneath the central crest of Yucca Mountain. Drill Hole USW G-3 was cored continuously from 2625.4 ft (800.2 m) to 5030.8 ft (1533.4 m) nearby. Studies of the mineralogy and petrology of these core samples concentrate on the products of low-temperature diagenetic alteration; they indicate less alteration, and of lower grade, than is noted in any of the cored drill holes from farther north at Yucca Mountain. Relatively unstable primary phases such as glasses, tridymite, and cristobalite are preserved to greater depth. Clinoptilolite persists to greater depth, and authigenic albite, a relatively high-grade secondary mineral, does not occur. Calcite is rare, and mordenite is virtually absent, except for rare occurrences along fractures in the Crater Flat Tuff. Compositional zonation of zeolites is highly variable and poorly correlated with depth, and a clearly defined smectite-to-illite transition is lacking. Smectite interstratifications, poor in illite, indicate a maximum alteration temperature no greater than 40 exp 0 C at the bottom of USW G-3 (5031 ft or 1533 m). All these features contrast sharply with those of samples from the northern part of Yucca Mountain. In particular, the tuff of Calico Hills can not be relied upon as a zeolitized sorptive barrier throughout Yucca Mountain. However, four commonly zeolitized intervals are defined and traced across the exploration block at Yucca Mountain. Analysis of these intervals indicates that equivalent thicknesses of 100% sorptive zeolite range from 24 to 78 m at various localities below any proposed repository in the moderately to densely welded Topopah Spring unit and above the static water level. 26 references, 12 figures, 4 tables. (ERA citation 09:039724)